The objectives of this technique are essentially the following: 1. To find the most efficient reduction conditions
3. To identify the supported precursor phases and their interactions with the support
4. To characterise complex systems, as bimetallic or doped catalyst, to determine the role of the second component and to establish alloy formation or promotion effects
There are several interesting studies about this technique: Robertson et Al. [17] first reported TPR profile of nickel and nickel-copper catalysts and since then many catalysts have been investigated. In the TPR technique an oxided catalyst precursor is submitted to a programmed temperature rise, while a reducing gas mixture is flowed over it (usually, hydrogen diluted in some inert gas as nitrogen or argon). In the TPO technique, the catalyst is in the reduced form and it is submitted to a programmed temperature increase, but in this case, an oxidising mixture of gas (oxygen in helium) is flowed over the sample. The reduction or oxidation rates are continuously measured by monitoring the change in composition of the reactive mixture of after the reactor. The decrease in H2 or O2 concentration in the effluent gas with respect to
the initial percentage monitors the reaction progress. An interesting application of this technique is that the TPR/O analysis may be used to obtain evidence for the interaction between the atoms of two metallic components, in the case of bimetallic system or alloy as already cited. In general, TPR/TPO studies are carried out under low partial pressure of the reactive gas. In this way it is possible to observe the intermediate reactions, depending from
TPD NH3 Mordenite Rate = 10°C/min, N2 Flow= 30cc/min
0,00E+00 5,00E+03 1,00E+04 1,50E+04 2,00E+04 2,50E+04 3,00E+04 3,50E+04 4,00E+04 0 100 200 300 400 500 600 700 800 900 Temperature (°C) Mi c ro v o lt/ g
analytical conditions as temperature rate, flow rate and concentration of reactive gas. The TPR/TPO methods are used for quantitative and quantitative analysis. In effect, the spectra produced are characteristic of a given solid. TPO is less commonly used then TPR, but the quantitative considerations for this type of analysis are more correct, in particular if the two analyses are performed in succession (hydrogen/oxygen titration). When used in combination, the two techniques can provide useful information in the study of the reactivity and redox behaviour of catalysts.
7.1 – Reduction and oxidation reactions
The reaction between a metal oxide MxOy and hydrogen, reducing the system to produce
the pure metal M is represented by the equation: MxOy(solid) + H2 M(solid) + H2O
In the thermodynamic point of view, the reduction of a solid oxide is feasible if the standard free energy change ∆G0 is negative. If ∆G0
is positive, the second term of the equation (28) must be sufficiently negative to make also negative ∆G:
∆G = ∆G° + RT log (PH2O / PH2) (28)
The reduction process is a bulk phenomenon and the degree of reduction (α as a function of time or temperature and hydrogen pressure) is interpreted in terms of mechanism by which the reduction occurs. Two different models can interpret the reduction processes: the nucleation model and the contracting sphere model. In the first case, according to nucleation mechanism, the reduction begins after some time and at a given temperature bringing to the formation of a solid product nucleus. During the nucleation, oxygen ions are removed from the lattice with progressive formation of solid metal and hydrogen and oxygen molecules diffuse at the interface oxide/metal/atmosphere. If the nucleation process is very fast, the real formation of separated and independent nuclei cannot be distinguished and the second mechanism takes place (contracting sphere model). The result during the reduction process, in this case, is a total coverage of the solid oxide particle with a tin layer of metallic product as an eggshell. In effect, the distinction between the two models is not only theory, but it has a consequence in the rate of reduction that is very different. In figure 11 is reported a graphic comparison between the different dependence of the degree of the reduction from the time [18]. The A diagram is relative to metal oxide reduction by a nucleation mechanism, while the diagram B reports the case of contracting sphere model.
Figure 11. Dependence of reduction degree from time.
In the first case, it is possible to identify a maximum rate; this profile is typical of auto- catalysed reactions. In the second case, the rate of reaction decreases continuously until the reaction process is completed as there is a continuous decreasing of the metal /oxide interface. It is common, in catalysis, to have a supported system that may exhibit a different reductive behaviour in comparison to unsupported metal oxides due to possible interactions between the metal and the support. The metal/support interactions may modify the reaction mechanism, promoting the atom diffusion on the surface of supported metal oxides or inhibiting the reduction process. This last is the case of cobalt supported on alumina, where cobalt aluminate, that is a system very difficult to be reduced, is formed. Similar possibility occurs in the case of bimetallic systems, where the second metallic compound (the doping species) may have a promoting effect by increasing the number of nucleation sites or providing a higher concentration of dissociated hydrogen that is transferred trough the support by the spillover effect. In the case of the TPO analysis, the reaction involved is an oxidation of a pre- reduced system:
xM + y/2 O2 MxOy
In the above reaction, water is not produced and the oxidation degree can be interpreted according to the same model of TPR. TPO analyses are often performed in combination with TPR. In this way, it is possible to obtain additional information about the metallic compounds in the catalyst active phase and it is possible to separate the contribution of different metallic species in multi-metallic systems. The combination of the two reactions is a real titration of the hydrogen/oxygen consumption, permitting the calculation of the metal phase percentage in the catalyst (of course if the stoichiometric factor of the reaction is known). Another advantage of combining the two analyses is that the TPO permits to remove undesired
contaminants then to concentrate the attention on the characterisation of the catalyst active phase.
7.2 – Experimental aspects
The experimental apparatus for TPR/TPO analyses is usually the same as the one used for TPD measurement (see 5.3.1). The fundamental difference is the type of carrier gas flowing trough the sample (see par. 6.2) and the pre-treatment procedure. Moreover, it is important to underline again that the TPR/TPO are analyses investigate the bulk system while TPD gives information about the surface behaviour of the catalyst.
7.2.1 – Sample preparation
The procedure to collect the TPR/TPO/TPD data is also comprehensive of the sample pre-treatment. Several types of procedure can be chosen in relation to the sample nature and type of information required. In fact, the diversification of the pre-treatment permits to obtain a wider range of parameters on a given catalyst. Generally, before starting a TPR analysis, the sample should be in its oxide form. The pre-treatment, in this case, consists in oxidising the catalyst in flow of pure oxygen or air, then flowing an inert gas to purge the product formed as water or carbon residues. Both pre-treatments must be effected at a given temperature to assure that the two processes are feasible. In case of TPO analysis, the sample must be preventively reduced to obtain the active metal in zero valence form. The standard pre- treatment is a reducing procedure effected at a given temperature (isothermal or increased by a constant rate). The pre-treatment procedure permits also to remove undesired compounds as residual solvent traces or products resulting from the precursor decomposition. Alternatively, it is possible to remove only the physisorbed water to obtain information on the efficiency of the activation procedure or on the poisoning phenomena of exhaust catalysts. The calcination operation is effected at high or medium temperature in flow of air to decompose the precursor compound. The precursor presence in fact can negatively influence the reducibility of the catalyst. In the case of cobalt supported on alumina, for example, if the calcination temperature necessary to decompose the precursor (generally cobalt nitrate) is too high cobalt aluminate is formed. The consequence is a decrease of the metal active surface. By changing the pre-treatment methods before the TPR or TPO analyses, it is possible to investigate other catalyst behaviours that are related to the temperature. For example, modifications of analytical profiles due to temperature variations in the pre-treatment permit to estimate effects as synterisation or other metal/support interactions. In the example of cobalt/alumina catalyst, this type of studies permitted to state the best pre-treatment procedure to avoid the formation of cobalt aluminate. The best reducibility of this supported metal is achieved by pre-treating the catalyst at temperatures below 375°C and by performing the calcination process in flow of pure hydrogen.
7.2.2 – Analytical method
During the TPR/TPO analyses, several products as water, CO or CO2 are formed. It is
important to remove all undesired gas molecules that can interfere in the signal output. A correct pre-treatment and the use suitable traps to stop secondary products are therefore necessary. The choice of the analytical parameters, in particular temperature and flow rates, is fundamental to obtain significant reaction profiles. The problem related to the difficulty in comparing different analyses has received little attention in literature because the conditions
of sample preparation, pre-treatment and acquisition of experimental data are often omitted. Delanay G. [19], for example, reported the demonstration that the experimental conditions affect the temperature at which the reduction occurs. In any case, all the experimental parameters as hydrogen or oxygen concentration in the gas mixture, temperature increasing rate, total flow rate, sample weight and contact time can make influence on the analytical profiles. These parameters have effect also on the detector sensitivity (i.e. the flow rate). Monti et Al. [20] proposed a method to standardise the TPR/TPO data defining a number k, given by:
k = S0 / (V* C0) (29)
where S0 is the hypothetical amount of initial reducible species in the sample expressed in
µmol, V*
C0 is the molar flow rate (µmol/s) of the reactive gas. This number should be in the
range 55-150 s to have accurate and reliable results from the TPR/TPO analysis and above all to have comparable data. A typical example is the TPR analysis of cupric oxide: changing the temperature and the flow rates of the analysis, two reaction profiles will result: the resolution of the analysis is changed and it is possible or to distinguish the two phases of the reduction process identified by two peaks (CuII CuI Cu 0 ) or to obtain only one peak comprehensive of the total hydrogen consumption that is involved in the two processes. In the second case, the advantage is to calculate more easily the total quantity of reacted gas. In general, when in the sample there is only one component is useful to perform the analysis with a low temperature rate to observe the mechanism of the reaction process. In the case of multi-metallic catalyst higher temperature rate permits to separate the different contribution of the reactive components [21].
7.3 – Quantitative calculation of reduced/oxided sites
When a reduction process is considered (similarly in the oxidation process), it is possible to express the rate of the reaction by the equation:
r = -d [ MxOy ] / dt = -d [ H2] / dt = k [ MxOy]p [ H2 ]q (30)
where k is a constant given by the Arrhenius equation k = A e-E/RT and dT = β dt, T is the temperature (K) and t is the time (min).
As temperature is increased linearly, for both TPR and TPO, it is possible to correlate the concentration variation of the reactive gas by:
d [ H2] /dt = - β d [ H2] / dT (31)
The possibility to correlate the parameters determining the reaction process (H2
concentration, temperature rate and time) and the kinetic-thermodynamic parameters confirms that the TPR/TPO data are very useful characterisation techniques. Experimental TPR/TPO data offer important information about the change rate of some parameters in function of the temperature. The system can be described as a rector by correlating reduction/oxidation profiles to kinetic/thermodynamic parameters. The consumption rate of the reactive gas r, is correlated to the flow rate φ, to the reactor element dx and the fraction of conversion df by the following expression:
r = φ df/dx (32)
7.4 – Evaluation of average metal oxidation degree
Temperature programmed reaction permits to estimate exactly the amount of reactive gas consumed during the reaction. This quantity is correlated to the oxided form of the sample, but it is necessary to follow several conditions:
1. An opportune pre-treatment of the sample must be carefully chosen to avoid secondary and undesired reactions.
2. The detection system must be correctly calibrated with standard samples or blank analysis to estimate exactly the amount of gas involved in the reaction.
3. Analytical parameters used during the measurement must guarantee that the reaction is thermodynamically feasible.
If all the above conditions are respected, the average metal oxidation degree can be measured if the metal percentage and the reaction stoichiometry are known. The degree of sample oxidation is given by the ratio:
α = nH / ( nm SF ) (33)
where nH is the number of detected hydrogen atoms that are proportional to the peak area, Nm
is the total number of metal atoms contained in the sample, Sf is the stoichiometric factor
depending by the initial oxidation state and by the final product.
7.5 – Analytical examples
In figure 12 is reported the overlay of TPR analyses carried out on four catalysts containing the 5%(wt) of cobalt supported on alumina. They have been prepared by wetness impregnation ad then doped with different percentage of iridium [22]. The pre-treatment procedure is the same for all the samples: the catalysts, pre-calcinated in air at 350°C, have successively been cleaned in N2 flow at 150°C and finally cooled at room temperature. The
TPR was carried out with a temperature rate of 10°C/min and a flow rate of 30 cc/min of a mixture of 5%H2/N2. There are two evidences in the TPR profiles: the H2consumption
increases when the percentage of doping metal (Ir) is increased while and the maximum temperature, related to the maximum consumption of gas, decreases accordingly. This example is a clear demonstration that the TPR analysis offers information about the reducibility of metallic samples and that it is possible to estimate quantitatively the effect due to the presence of a second metallic species. Multi-metallic systems are known for the difficulty in their characterisation.
Figure 12. TPR profiles on 5% cobalt on alumina with different doping percentage of iridium. Gas used 5% hydrogen in nitrogen, flow 30 cc/min, rate 10 C/min (TPDRO 1100).
In figure 13 is reported a typical reduction profile of pure cupric oxide. Cupric oxide can be conveniently used to calibrate the detector signal. Sharp reduction peaks permits a better integration and a correct calculation of the reacted hydrogen. This result can be achieved by using a relatively high temperature rate (15 °C/min) and a small amount of sample (20-30 mg).
Figure 13:. TPR profile on CUO using 5% hydrogen in nitrogen (TPDRO 1100).
TPR CuO
5% H2 in N2, Flow=30 cc/min, Rate=15°C/min.
0,00E+00 2,00E+06 4,00E+06 6,00E+06 8,00E+06 1,00E+07 1,20E+07 0 100 200 300 400 500 600 700 800 900 Temperature (°C) M ic rov ol t/ g